A charger is used for charging the surface of a photoconductor to a predetermined potential as an initial image forming process in electrophotographic image forming devices such as photocopiers, facsimiles and laser printers. Such a charger used in general is a contact charging device having a charging roller (e.g. a rubber roller) which contacts the surface of the photoconductor while applying a voltage to the photoconductor through the charging roller.
FIG. 8 shows a state of a process for charging the photoconductor by a roller-type contact charging device. As shown in FIG. 8, the contact charging device is made up of a charging roller 10 and a direct-current (dc) low voltage power source 2, the charging roller 10 having a core 10a of a cylindrical shape as a center and being covered with an elastic member (charging member) 10b which is made of conductive rubber, etc. of a hollow cylinder, and the charging roller 10 coming into contact with a photoreceptor drum 3 at a nip portion (contact portion). The photoreceptor drum 3 is made up of a photoconductor 3b formed over a drum body 3a which is made of metal of a hollow cylinder.
The dc low voltage power source 2 applies a dc voltage E between the core 10a of the charging roller 10 and the drum body 3a of the photoreceptor drum 3 which is grounded. Accordingly, an inner peripheral surface (hereinafter referred to as inner surface) of the elastic member 10b is set to have a negative potential, and an inner surface of the photoconductor 3b a ground potential. When the photoreceptor drum 3 is driven to rotate in a direction of arrow A, the charging roller 10 rotates about the central axis of the core, in a direction of arrow B, following the rotation of the photoreceptor drum 3. Therefore, the surface of the photoconductor 3b which is brought into contact with the surface of the elastic member 10b at the entrance of the nip portion is charged while passing the nip portion, thus inducing a potential change.
Referring to FIG. 8, a power source is the dc low voltage power source 2, and the surface of the photoconductor 3b is negatively charged by having the drum body 3a grounded while making the core 10a of the charging roller 10 to have a negative potential. Alternatively, the dc voltage is applied so that a core side of the charging roller is set to have a higher potential with respect to the drum body so as to make a charge potential of the photoconductor a positive polarity. Further alternatively, the power source is an alternating-current (ac) superimposed power source in which an ac component is superimposed on a dc component, and the ac superimposed power source applies a voltage which varies as a function of time.
As shown in FIG. 8, the elastic member 10b can be regarded as a set of micro-regions whose resistance and electrostatic capacity are equivalent to one another and which are generated by being divided by infinite numbers of division lines in a radial direction linking the inner surface (surface on the side of a rotational center) and the outer surface. Each micro-region is equivalently represented by a parallel circuit made up of resistance R per unit area and electrostatic capacity C per unit area, the resistance R being obtained by multiplying a resistance value measured between predetermined regions of the inner surface and the outer surface of the elastic member 10b, respectively, by an area measured on the side of the outer surface, the electrostatic capacity C being obtained by dividing the electrostatic capacity measured between the inner and outer surfaces, by an area of the outer surface. In addition, the photoconductor 3b can be regarded as a set of infinite numbers of micro-regions in which a spacing between the inner surface (surface on the side of the rotational center) and the outer surface is equivalently represented by electrostatic capacity C.sub.0 per unit area.
FIG. 9 shows an equivalent circuit when the photoconductor 3b is charged by the contact at the nip portion between a surface of the micro-region of the elastic member 10b and a surface of the micro-region of the photoconductor 3b. In the foregoing arrangement of the contact charging device, a power voltage e(t) shown in FIG. 9 is equal to a dc voltage E. In this case, it is assumed that the charging roller 10 and the photoreceptor drum 3 are rotating at the same circumferential speed without slipping with each other at the nip portion.
The equivalent circuit shown in FIG. 9 is formed when the micro-region of the elastic member 10b and micro-region of the photoconductor 3b shown in FIG. 8 come into contact with each other upon reaching the entrance of the nip portion, and the dc voltage E is fed to the equivalent circuit, which starts charging the photoconductor 3b, i.e. charging the electrostatic capacity C.sub.0. After that, as the micro-regions move toward an exit of the nip portion, a charge current flows into the electrostatic capacity C.sub.0 in accordance with a time constant C.sub.0.multidot.R (C is small and negligible) which is determined by the resistance R of the elastic member 10b and the electrostatic capacity C.sub.0 of the photoconductor 3b. This results in increase in a terminal voltage e.sub.c (t) of the electrostatic capacity C.sub.0. The charge current from the elastic member 10b toward the photoconductor 3b is equivalent of injecting negative charge, and is maximum at the entrance of the nip portion, then, decreases toward the exit. Consequently, a potential distribution at the nip portion (surface of the photoconductor 3b) takes the form substantially as shown in FIG. 8. Here, V.sub.0 is an initial potential on the surface of the photoconductor 3b.
The elastic member 10b is required to be of a characteristic which would cause the photoconductor 3b to be uniformly charged at the end. It is known that uniformity of charge over the photoconductor 3b can be improved by making the time constant sufficiently small by reducing the resistance R of the elastic member 10b with respect to a given photoconductor 3b.
However, adopting only the foregoing method that attempts to improve the charge uniformity by reducing the time constant at the time of charging the photoconductor 3b raises a problem. Namely, sufficient charge uniformity cannot be obtained due to restrictions on a setting range of the time constant, which are imposed by adapting to pinhole leakage of the photoconductor 3b or by a nip width which can be set.
Japanese Examined Patent Publication No. 92617/1995 (Tokukohei 7-92617 published on Oct. 9, 1995; corresponding to U.S. Pat. No. 5,126,913) discloses another method of obtaining a resistance value of a charging roller for performing uniform charging by means of a charge model of a photoconductor which employed resistance and electrostatic capacity of the photoconductor and the charging roller. In this charge model, however, the electrostatic capacity of the charging roller is used as a constant value. As discussed, since the elastic member 10b of the charging roller 10 and the photoconductor 3b rotate while keeping contact with each other, their contact face (nip portion) is renewed constantly. Therefore, the surface of each micro-region of the elastic member 10b supplies the micro-region of the photoconductor 3b with charge whenever it contacts the surface of the micro-region of the photoconductor 3b, which results in a potential change substantially as shown in FIG. 8. In addition, a current does not flow anywhere except at the nip portion during rotation, and therefore, it can be said that the surface of the micro-region of the elastic member 10b and the core thereof are at the equivalent potential except at the nip portion.
In this way, a charging operation for charging an arbitrary micro-region of the photoconductor 3b through the charging roller 10 is a repetition of intermittent application of a voltage, and a potential immediately before leaving the nip portion (time t.sub.0) is the charge potential of the photoconductor. Therefore, as shown in FIG. 10 (top), the power voltage e(t) in the equivalent circuit of FIG. 9 rises during nip portion passing time t.sub.0 and is equivalent to a rectangular pulse whose period is the rotation period T of the charging roller 10. Here, the terminal voltage e.sub.R (t) of each micro-region of the elastic member 10b becomes a waveform pulse shown in FIG. 10 (second from the top), charge current i(t) which flows from each micro-region of the elastic member 10b to the micro-region of the photoconductor 3b a waveform pulse shown in FIG. 10 (third from the top), and the terminal voltage e.sub.c (t) a waveform pulse shown in FIG. 10 (bottom).
More specifically, a voltage applied across a combined region of the micro-region of the elastic member 10b and the micro-region of the photoconductor 3b which is in contact therewith has a frequency component (ac component). Consequently, the electrostatic capacity C of the elastic member 10b varies depending on a frequency. The frequency component varies depending on various conditions such as a roller diameter of the charging roller 10 and the nip width. Thus, in the foregoing method disclosed in the above publication which does not take into account frequency characteristics of the electrostatic capacity C, charge uniformity is not yet sufficiently improved because the resistance value of the charging roller 10 is not optimized.